Thursday, 17 July 2014

I liked this article summarizing the current state of science regarding multiverse theories. It's very clear and well-illustrated, and, as far as I know, accurate.

This quote is particularly interesting:

So as appealing as the idea is that there are other Level 1 Multiverses out there with different constants than our own, we have good physical reasons based on observable evidence to think it’s unlikely, and zero good reasons (because wanting it to be so is not a good reason) to think it’s likely.

He doesn't mention why anyone would "want it to be so", i.e. believe that other universes of a "Level 1 Multiverse" could have different constants to our own. However, I'm pretty sure he had in mind the selection-bias explanation for the anthropic coincidences. That is, if we accept that only a narrow range of possible values for the fundamental physical constants are compatible with the existence of intelligent life (and most scientists do, I think), then we would like to be able to explain why our universe's constants are in that range. If there are an abundance of different universes, each with different values for the physical constants, then most of them would be dead but a lucky few would sustain intelligent life, and naturally we can only observe one of those latter.

This reasoning relies on the assumption that there are abundance of different universes with different values for the physical constants. Scientists obviously would prefer to be able to deduce this from observations rather than pull it out of thin air. As discussed in the above article, theories of chaotic inflation --- which are reasonably well-grounded in observations of our own universe --- predict the existence of alternate universes. If those universes could have different values for physical constants (or even different physical laws), we'd have an observationally-grounded theory that predicts exactly the kind of multiverse needed to power the selection-bias explanation for the anthropic coincidences. Unfortunately for proponents of that explanation, the science isn't working out.

Of course, the selection-bias explanation could still be valid, either because new information shows that chaotic-inflation universes can get different constants after all, or because we assume another level of multiverse, whose existence is not due to chaotic inflation. However, many scientists (such as in the article above) find the assumption of a higher-level multiverse quite unsatisfactory.

Unsurprisingly, I'm comfortable with the explanation that our universe was intentionally created for intelligent life to live in. Incidentally, you don't need to be a classical theist to adopt this explanation; some atheist philosophers argue with varying degrees of seriousness that we are (probably) living in a simulation.

Thursday, 3 July 2014

Sticky headers that scroll normally until they would scroll out of view and then stop moving

Panels that slide laterally as you scroll vertically

Elements that shrink as their available space decreases instead of scrolling out of view

Scrollable panels that resist scrolling as you get near the end

Obviously we need to support these behaviors well on the Web. Also obviously, we don't want to create a CSS property for each of them. Normally we'd handle this diversity by exposing a DOM API which lets developers implement their desired behavior in arbitrary Javascript. That's tricky in this case because script normally runs on the HTML5 event loop which is shared with a lot of other page activities, but for smooth touch tracking these scrolling animation calculations need to be performed reliably at the screen refresh rate, typically 60Hz. Even for skilled developers, it's easy to have a bug where once in a while some page activity (e.g. an event handler working through some unexpected large data set) blows the 16ms budget to make touch dragging less than perfect, especially on low-end mobile devices.

There are a few possible approaches to fixing this. One is to not provide any new API, hope that skilled developers can avoid blowing the latency budget, and carefully engineer the browser to minimize its overhead. We took this approach to implementing homescreen panning in FirefoxOS. This approach sounds fragile to me. We could make it less fragile by changing event dispatch policy during a touch-drag, e.g. to suppress the firing of "non-essential" event handlers such as setTimeouts, but that would add platform complexity and possibly create compatibility issues.

Another approach would be to move scroll animation calculations to a Worker script, per an old high-level proposal from Google (which AFAIK they are not currently pursuing). This would be more robust than main-thread calculations. It would probably be a bit clumsy.

Another suggestion is to leverage the Web's existing and proposed animation support. Basically we would allow an animation on an element to be use another element's scroll position instead of time as the input to the animation function. Tab Atkins proposed this with declarative CSS syntax a while ago, though it now seems to make more sense as part of Web Animations. This approach is appealing because this animation data can be processed off the main thread, so these animations can happen at 60Hz regardless of what the main thread is doing. It's also very flexible; versions of all of the above examples can be implemented using it.

One important question is how much of the problem space is covered by the Web Animations approach. There are two sub-issues:

What scrolling effects depend on more than just the scroll position, e.g. scroll velocity? (There certainly are some, such as headers that appear when you scroll down but disappear when you scroll up.)

For effects that depend on just the scroll position, which ones can't be expressed just by animating CSS transforms and/or opacity as a function of the scroll position?

If people are aware of scrolling effects in either of those two categories, it would be very useful to hear about them.